Lithographic apparatus and device manufacturing method

ABSTRACT

A method and apparatus make use of data representing changes in wavelength of a radiation source to provide control of focal plane position or to provide correction of sensor data. In the first aspect, the wavelength variation data is provided to control systems that control focus by moving apparatus components including, for example, the mask table, the substrate table or optical elements of the projection optical system. In the second aspect, variation data is used in correcting, e.g., focal plane position data measured by a transmitted image sensor. The two aspects may be combined in a single apparatus or may be used separately.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a lithographic apparatus and a method for manufacturing a device.

2. Background of the Related Art

A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a method of imaging a pattern onto a substrate using a lithographic apparatus including patterning a radiation beam, measuring a wavelength variation of the radiation beam, projecting the patterned beam onto the substrate, and adjusting a focal plane of the projecting based on the measured wavelength variation.

According to another aspect of the invention, there is provided a method of correcting a projected image measurement within a lithographic apparatus including projecting a patterned radiation beam using a projection optical system, measuring a wavelength variation of the radiation beam, measuring image information of the patterned beam, and correcting the measured image information based on the measured wavelength variation.

According to another aspect of the invention, there is provided a lithographic apparatus comprising an illumination system configured to condition a radiation beam, a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam, a substrate table constructed to hold a substrate, and a projection system configured to project the patterned radiation beam onto a target portion of the substrate, a sensor, constructed and arranged to measure a wavelength variation of the radiation beam, and a controller, configured and arranged to adjust a focal plane of the lithographic projection apparatus based at least in part on wavelength variation measurements of the sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

FIG. 1 depicts a lithographic apparatus according to an embodiment of the invention;

FIG. 2 is a schematic diagram of a system in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition         a radiation beam B (e.g., UV radiation or DUV radiation).     -   a support structure (e.g., a mask table) MT constructed to         support a patterning device (e.g., a mask) MA and connected to a         first positioner PM configured to accurately position the         patterning device in accordance with certain parameters;     -   a substrate table (e.g., a wafer table) WT constructed to hold a         substrate (e.g., a resist-coated wafer) W and connected to a         second positioner PW configured to accurately position the         substrate in accordance with certain parameters; and     -   a projection system (e.g., a refractive projection lens system)         PS configured to project a pattern imparted to the radiation         beam B by patterning device MA onto a target portion C (e.g.,         comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

The support structure supports, i.e., bears the weight of, the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system.”

The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the support structure (e.g., mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in FIG. 1) can be used to accurately position the mask MA with respect to the path of the radiation beam B, e.g., after mechanical retrieval from a mask library, or during a scan. In general, movement of the mask table MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short-stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the following modes:

1. In step mode, the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the mask table MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.

3. In another mode, the mask table MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

While in principle lasers produce a narrow bandwidth, constant wavelength pulse of radiation, in practice, there is a minor variation in wavelength from pulse to pulse. By variation in wavelength is meant both variation in the peak wavelength within a given wavelength range (i.e., a change in the shape of the spectrum) and a wavelength shift of the entire spectrum (i.e., a change in the position of the spectrum). Either type of drift tends to result in a change in the centerline wavelength of the spectrum. Lasers designed for lithographic systems include control loops that allow the laser output to have centerline drifts on the order of a fraction of a picometer or even less. For a laser producing light at 193 nm, for example, this represents a drift of a few parts in 1,000,000. Nonetheless, because lithographic projection lenses are designed to be corrected for a specific wavelength, and retain some degree of chromaticity, even this small variation can result in slight differences in focus depending on the pulse wavelength variation of the laser light. Thus, it would be useful to correct for wavelength variation between pulses in order to improve imaging focus without requiring a more chromatically robust lens design. Likewise, the present invention can find application in correcting for drift in a system using a continuous wave source.

At a typical pulse frequency of 1000-2000 Hz, it follows that pulse to pulse wavelength changes occur on the sub-millisecond time scale. In general, projection lenses are not designed to be re-focused on a pulse by pulse basis and, in particular, the response time of focus control is generally insufficient to modify focus at kilohertz frequencies. On the other hand, a phenomenon known as wavelength chirp appears to occur in a time frame on the order of a few milliseconds (e.g., 3-10 ms). At a typical pulse rate of 1000-2000 Hz, this translates to between three and 20 pulses of the laser. Thus, the response time of focus control mechanisms may be short enough to allow correction over the periodicity of the chirp phenomenon.

Referring now to FIG. 2, a control loop in accordance with the present invention is shown. The light source, e.g., laser 10, includes an internal control loop 12 that is designed to maintain wavelength stability. Because the control loop 12 is designed to control the laser output on a pulse to pulse basis, the laser 10 includes an element 14 for measuring pulse to pulse wavelength variation. As shown in FIG. 2, in accordance with this embodiment, the wavelength measuring element 14 additionally provides that data to the scanner 20. As noted above, the scanner 20 will not, in general, be capable of adjusting focus with sufficient speed. The data received by the scanner 20 is then used in a feedback or feedforward control algorithm for controlling the scanner 20 focus. As should be understood by one of ordinary skill, in the event that the response time of the focus controls of the scanner 20 is short enough to allow for a pulse to pulse wavelength correction, then the present invention can be implemented on a pulse to pulse basis.

The scanner focus can be controlled in a number of ways. For example, the plane of the patterning device or the substrate holder may be adjusted along the optical axis of the scanner using the positioners PM, PW. Alternately, individual lens elements or lens groups forming a part of the projection system PS may be actuated, for example by piezoelectric elements configured for that purpose. In an another alternate embodiment, the substrate table can be locally adjusted relative to the focal plane in order to adapt the focus. Additional focal control mechanisms will be readily apparent to those of ordinary skill in the art and should be understood to within the scope of the present invention.

An additional effect of pulse to pulse instability arises in the operation of inboard sensor systems that make use of the projection light. For example, the transmitted image sensor (TIS) as incorporated in certain lithography equipment makes measurements of the image transmitted by the projection system PS. It makes these measurements at or near the plane of the substrate table WT and is thus subject to inaccuracies resulting from focal plane drift due to wavelength variation of the source. Thus, in a second embodiment of the present invention, pulse variation data is fed to the scanner 20 and used in correction of TIS information. In this regard, the data may be directly fed to the TIS and/or its respective processor in a feedforward or real-time correction mode. Alternately, it may be fed separately to a processor for post-processing of the TIS data, taking into account the pulse variation data.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion,” respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g., semiconductor memory, magnetic or optical disk) having such a computer program stored therein.

The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below. 

1. A method of imaging a pattern onto a substrate using a lithographic apparatus comprising: patterning a radiation beam; measuring a wavelength variation of the radiation beam; projecting the patterned beam onto the substrate; and adjusting a focal plane of the projecting based on the measured wavelength variation.
 2. A method as in claim 1, wherein the measuring comprises measuring a centerline drift of a spectrum of the radiation beam.
 3. A method as in claim 2, wherein the measuring comprises measuring variation between pairs of consecutive pulses of the radiation beam.
 4. A method as in claim 2, wherein the measuring comprises measuring wavelength chirp of the radiation beam.
 5. A method as in claim 1, wherein the adjusting comprises changing a position of a patterning device used in the patterning along an optical axis of the lithographic apparatus.
 6. A method as in claim 1, wherein the adjusting comprises changing a position of the substrate along an optical axis of the lithographic apparatus.
 7. A method as in claim 1, wherein the adjusting comprises changing a position of one or more optical elements of a projection optical system used in the projecting.
 8. A method as in claim 1, further comprising: measuring image information of the patterned beam; and correcting the measured image information based on the measured wavelength variation.
 9. A method as in claim 8, wherein the image information is image quality information measured at or near to a substrate plane of the lithographic apparatus.
 10. A method as in claim 9, wherein the image information is a location of a focal plane of the image.
 11. A method as in claim 10, wherein the image information is measured with a transmitted image sensor.
 12. A method of correcting a projected image measurement within a lithographic apparatus comprising: projecting a patterned radiation beam using a projection optical system; measuring a wavelength variation of the radiation beam; measuring image information of the patterned beam; and correcting the measured image information based on the measured wavelength variation.
 13. A method as in claim 12, wherein the measuring comprises measuring a centerline drift of a spectrum of the radiation beam.
 14. A method as in claim 13, wherein the measuring comprises measuring variation between pairs of consecutive pulses of the radiation beam.
 15. A method as in claim 13, wherein the measuring comprises measuring wavelength chirp of the radiation beam.
 16. A method as in claim 12, wherein the image information is image quality information measured at or near to a substrate plane of the lithographic apparatus.
 17. A method as in claim 16, wherein the image information is a location of a focal plane of the image.
 18. A method as in claim 17, wherein the image information is measured with a transmitted image sensor.
 19. A lithographic projection apparatus comprising: an illumination system configured to condition a radiation beam; a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; a projection system configured to project the patterned radiation beam onto a target portion of the substrate; a sensor, constructed and arranged to measure a wavelength variation of the radiation beam; and a controller, configured and arranged to adjust a focal plane of the lithographic projection apparatus based at least in part on wavelength variation measurements of the sensor.
 20. A lithographic projection apparatus according to claim 19 wherein the controller is configured to adjust the focal plane by adjusting a position of the substrate table.
 21. A lithographic projection apparatus according to claim 19 wherein the controller is configured to adjust the focal plane by adjusting a position of the support.
 22. A lithographic projection apparatus according to claim 19 wherein the controller is configured to adjust the focal plane by adjusting a position of at least one element of the projection system.
 23. A lithographic projection apparatus according to claim 19 wherein a source of the radiation beam comprises the sensor.
 24. A lithographic projection apparatus according to claim 19, further comprising a sensor, configured and arranged to measure image information proximate a plane of the substrate and wherein the wavelength variation measurements are used to correct measured image information.
 25. A lithographic projection apparatus according to claim 24, wherein the image information comprises a position of the focal plane. 